1. Field of the Invention
The present invention relates to oxidative desulfurization processes to efficiently reduce the sulfur content of hydrocarbons, and more particularly an oxidative desulfurization process using nitrous oxide as a gaseous oxidant to produce hydrocarbons products including fuels having ultra low sulfur content.
2. Description of Related Art
The discharge into the atmosphere of sulfur compounds during processing and end-use of the petroleum products derived from sulfur-containing sour crude oil pose health and environmental problems. The stringent reduced-sulfur specifications applicable to transportation and other fuel products have impacted the refining industry, and it is necessary for refiners to make capital investments to greatly reduce the sulfur content in gas oils to 10 parts per million by weight (ppmw), or less. In industrialized nations such as the United States, Japan and the countries of the European Union, refineries for transportation fuel have already been required to produce environmentally clean transportation fuels. For instance, in 2007 the United States Environmental Protection Agency required the sulfur content of highway diesel fuel to be reduced 97%, from 500 ppmw (low sulfur diesel) to 15 ppmw (ultra-low sulfur diesel). The European Union has enacted even more stringent standards, requiring diesel and gasoline fuels sold in 2009 to contain less than 10 ppmw of sulfur. Other countries are following in the direction of the United States and the European Union and are moving forward with regulations that will require refineries to produce transportation fuels with an ultra-low sulfur level.
To keep pace with recent trends toward production of ultra-low sulfur fuels, refiners must choose among the processes or crude oils that provide flexibility to ensure that future specifications are met with minimum additional capital investment, in many instances by utilizing existing equipment. Conventional technologies such as hydrocracking and two-stage hydrotreating offer solutions to refiners for the production of clean transportation fuels. These technologies are available and can be applied as new grassroots production facilities are constructed. However, many existing hydroprocessing facilities, such as those using relatively low pressure hydrotreaters were constructed before these more stringent sulfur reduction requirements were enacted and represent a substantial prior investment. It is very difficult to upgrade existing hydrotreating reactors in these facilities because of the comparatively more severe operational requirements (i.e., higher temperature and pressure conditions) to obtain clean fuel production. Available retrofitting options for refiners include elevation of the hydrogen partial pressure by increasing the recycle gas quality, utilization of more active catalyst compositions, installation of improved reactor components to enhance liquid-solid contact, the increase of reactor volume, and the increase of the feedstock quality.
There are many hydrotreating units installed worldwide producing transportation fuels containing 500-3000 ppmw sulfur. These units were designed for, and are being operated at, relatively mild conditions, i.e., low hydrogen partial pressures of 30 kilograms per square centimeter for straight run gas oils boiling in the range of 180° C.-370° C.
However, with the increasing prevalence of more stringent environmental sulfur specifications in transportation fuels mentioned above, the maximum allowable sulfur levels are being reduced to no greater than 15 ppmw, and in some cases no greater than 10 ppmw. This ultra-low level of sulfur in the end product typically requires either construction of new high pressure hydrotreating units, or a substantial retrofitting of existing facilities, e.g., by integrating new reactors, incorporating gas purification systems, reengineering the internal configuration and components of reactors, and/or deployment of more active catalyst compositions. Each of these options represents a substantial capital investment.
Sulfur-containing compounds that are typically present in hydrocarbon fuels include aliphatic molecules such as sulfides, disulfides and mercaptans, as well as aromatic molecules such as thiophene, benzothiophene and its alkylated derivatives, and dibenzothiophene (DBT) and its alkyl derivatives such as 4,6-dimethyl-dibenzothiophene (DMDBT).
The economical removal of refractory sulfur-containing compounds is therefore exceedingly difficult to achieve, and accordingly removal of sulfur-containing compounds in hydrocarbon fuels to an ultra-low sulfur level is very costly by current hydrotreating techniques. When previous regulations permitted sulfur levels up to 500 ppmw, there was little need or incentive to desulfurize beyond the capabilities of conventional hydrodesulfurization, and hence the refractory sulfur-containing compounds were not targeted. However, in order to meet the more stringent sulfur specifications, these refractory sulfur-containing compounds must be substantially removed from hydrocarbon fuels streams.
Relative hydrodesulfurization reactivities and activation of sulfur compounds are shown in the below table:
TABLE 1NameDBT4-methy-DBT4,6-DMDBTStructure Reactivity k@250, s-157.710.41.0Reactivity k@300, s-17.32.51.0Activation Energy28.736.153.0Ea, Kcal/mol
Relative reactivities of sulfur compounds based on their first order reaction rates at 250° C. and 300° C., and 40.7 Kg/cm2 hydrogen partial pressure over Ni—Mo/Alumina catalyst are given (Steiner, P. et al., “Catalytic hydrodesulfurization of a light gas oil over a NiMo catalyst: kinetics of selected sulfur components,” Fuel Processing Technology, Vol. 79, Issue 1, Aug. 20, 2002, pages 1-12) in Table 1. DBT is 57 times more reactive than the refractory 4,6-DMDBT at 250° C. The relative reactivity decreases with increasing operating severity. With a 50° C. temperature increase, the relative reactivity of di-benzothiophene compared to 4,6-DMDBT decreases to 7.3 from 57.7.
The development of non-catalytic processes for desulfurization of petroleum distillate feedstocks has been widely studied, and certain conventional approaches are based on oxidation of sulfur-containing compounds described, e.g., in U.S. Pat. Nos. 5,910,440, 5,824,207, 5,753,102, 3,341,448 and 2,749,284.
Oxidative desulfurization is attractive for several reasons. First, conventional liquid phase oxidative desulfurization can occur at temperatures ranging from room temperature up to 200° C. and pressures ranging from 1 up to 15 atmospheres, thereby resulting a priori in reasonable investment and operational costs, especially compared to hydrogen consumption in hydroprocessing techniques which is usually expensive. Another attractive aspect of the oxidative process is related to the reactivity of aromatic sulfur-containing species. This is evident since the high electron density at the sulfur atom caused by the attached electron-rich aromatic rings, which is further increased with the presence of additional alkyl groups on the aromatic rings, will favor its electrophilic attack as shown in Table 2 (Otsuki, S. et al., “Oxidative desulfurization of light gas oil and vacuum gas oil by oxidation and solvent extraction,” Energy Fuels 14:1232-1239 (2000)). However, the intrinsic reactivity of molecules such as 4,6-DMDBT is substantially higher than that of DBT, which is much easier to desulfurize by hydrodesulfurization.
TABLE 2Electron Density of selected sulfur speciesK (L/(mol·Sulfur compoundFormulasElectron Densitymin))Thiophenol5.9020.270 Methyl Phenyl Sulfide5.9150.295 Diphenyl Sulfide5.8600.156 4,6-DMDBT5.7600.0767 4-MDBT5.7590.0627 DBT5.7580.0460 Benzothiophene5.7390.00574 2,5-Dimethylthiophene5.716— 2-methylthiophene5.706— Thiophene5.696—
Gondal et al. US 2008/0110802 discloses the removal of DMDBT by photoexciting atomic or molecular oxygen to a singlet or triplet energy state, mixing the photoexcited oxygen with the hydrocarbon fuel, and irradiating the hydrocarbon fuel with UV radiation from a tunable laser source at a wavelength corresponding to an absorption band of DMDBT. N2O is listed as one of the possible sources of oxygen. Laser-induced photolyzation of N2O is required in order to produce sufficient quantities of reactive oxygen species to promote oxidative desulfurization, which can be more costly and less efficient than other existing oxidative desulfurization processes. In addition, use of chemical catalysts are not disclosed by Gondal et al.
Darian et al. U.S. Pat. No. 4,746,420 discloses a process for decreasing the sulfur content and increasing the cetane number of a diesel oil. This process includes a first step in which diesel oil is contacted with nitrogenous agents followed with a liquid extraction aiming at removing sulfur containing impurities, instability causing compounds, Ramsbottom carbon, cetane depressing compounds and aromatic compounds. Although nitrogenous compounds are cited as reacting agents (including nitrous oxide) in the first step of process described in the Darian et al. reference, the goal of using such compounds is the promotion of nitration and esterification reactions of diesel oil, rather than oxidation of sulfur compounds. Moreover, the Darian et al. reference clearly teaches away from the use of oxidation catalysts.
Kocal U.S. Pat. No. 6,277,271 relates to a process for the desulfurization of oil comprising a first step of hydrodesulfurization followed by an oxidation step and finally an extraction step. While the Kocal reference lists nitrogen oxide broadly as potential oxidant in the oxidation step, the working examples only show use of peroxides in conjunction with an oxidant gas and with acetic acid. In addition, the Kocal reference does not disclose the use of inorganic catalysts.
None of the above-mentioned references describe an efficient and efficacious process for catalytic oxidative desulfurization using a gaseous oxidant.
Accordingly, it is an object of the present invention to desulfurize a hydrocarbon feedstock utilizing efficient gas phase oxidation thereby minimizing aqueous handling and removal requirements.